Researchers have found a Tyrannosaurus rex that was pregnant when it died some 68 million years ago – a discovery that could help paleontologists figure out how to distinguish between male and female therapod dinosaurs.

"It's a dirty secret, but we know next to nothing about sex-linked traits in extinct dinosaurs," said study co-author Lindsay Zanno, head of the Paleontology Research Lab at the North Carolina Museum of Natural Sciences, in a press release.

"Dinosaurs weren't shy about sexual signaling, all those bells and whistles, horns, crests, and frills, and yet we just haven't had a reliable way to tell males from females," she said. "Just being able to identify a dinosaur definitively as a female opens up a whole new world of possibilities."

This dinosaur wasn't found with a fetus, so how could researchers tell she was a mom-to-be? Inside the animal's fossilized thighbone was sex-specific reproductive tissue that's associated with egg shell-building in pregnant birds, the only living dinosaurs.

Lead author Mary Schweitzer, who is also a paleontologist at the museum and North Carolina State University, first spotted this specimen's medullary bone, as this reproductive tissue is called, over a decade ago.

But, as Dr. Zanno tells The Christian Science Monitor in an interview, the spongy-looking tissue can look like other things, including osteoporotic bones. So Dr. Schweitzer, Zanno, and their team looked for a way to determine conclusively that this strange feature is indeed fossilized reproductive tissue.

And they found one. Chemical analysis, or "a chemical pregnancy test," as Zanno calls it, confirmed that the tissue was medullary bone, which the team reports in a paper published Tuesday in the journal Scientific Reports.

"Finding this tissue is one of the only ways to tell if you have a female dinosaur, so identifying it correctly is really important," writes Stephen Brusatte, a paleontologist at the University of Edinburgh who was not part of this study, in an email to the Monitor. "It is great to see a wealth of new tools and methods used to confirm that this really unusual tissue only known in modern female birds was present in some [extinct] dinosaurs."

So what is a medullary bone, anyway?

When a bird is pregnant, a medullary bone forms inside her femur to serve as a source of calcium, so she can build strong shells for her eggs without leeching calcium from her other bones.

Birds are technically theropod dinosaurs like tyrannosaurs, but scientists weren't sure until now if the prehistoric dinosaurs would have the same adaptation.

"Crocodiles are the closest living relatives of dinosaurs, and birds are the only living dinosaurs. So when we want to know what dinosaurs were like, we often turn to these two living representatives," Zanno says. But crocodiles don't lay down medullary bone, she says. "Because their reproductive biology is so different, we have a hard time figuring out where dinosaurs fit on that spectrum."

As it turns out, "medullary bone is actually an ancient adaptation that preceded the evolution of birds," she says.

How can this find help with dino sex-typing?

The medullary bone itself is fleeting, so that feature may not the best way to determine whether a theropod is male or female.

"Birds begin to lay it down right before they start laying their eggs and then they use it up as they're laying their eggs. By the time they're done, it's gone," Zanno explains. In a T. rex, that window may last just two to four weeks.

"If we don't find it in the skeleton, that doesn't mean it's a male," she says. "It could just be a female that wasn't laying at the time."

But since the medullary bone does definitively identify this particular specimen as female, it can help researchers identify other sex-specific features. Zanno says she and her colleagues are "fortunate" to have the skull, parts of the pelvis, and some limbs of this egg-laying dinosaur. The skull and the pelvis are most likely to have differences between male and female dinosaurs, she says.

And if researchers can find a medullary bone in other dinosaur specimens, this comparison can be strengthened and expanded across other species of dinosaur.